The present invention relates to an improvement in the process for lost foam casting of metal parts in particular based on aluminum and alloys thereof.
It is known to those skilled in the art, for example from the teaching of U.S. Pat. No. 3,157,924, to effect casting of metals by using patterns of a foam of organic material such as polystyrene which are immersed in a mold formed by dry sand containing no binding agent. In an industrial context, such patterns are generally covered with a film of refractory material which is intended to improve the quality of the cast parts. In such a process the metal to be cast, which has been previously melted, is brought into contact with the pattern by way of a feed orifice and ducts which pass through the sand, and progressively replaces the pattern by burning it and transforming it primarily into vapors which escape between the grains of sand.
In comparison with the conventional casting procedure in a non-permanent mold, the process involving compacting and agglomeration of refractory materials in powder form eliminates the necessity of rigid molds which are associated with cores in a more or less complicated fashion by way of ducts, and permits easy recovery of the cast parts as well as easy recycling of the casting materials. It is therefore simpler and more economical than the conventional procedure. Moreover, it affords the designers of cast parts a greater degree of freedom as regards the shape of the parts. It is for that reason that that procedure has been found to be an increasingly attractive proposition from the industrial point of view.
However, it is handicapped by a number of disadvantages, two of these arising out of conventional metallurgical mechanisms, namely:
the relatively slow rate of solidification, which favors the formation of gassing pits resulting from hydrogen dissolved in the liquid aluminum alloy; and
the relatively slight thermal gradients, which favor the formation of micro-size shrinkage holes.
On the other hand, two other disadvantages arise out of mechanisms which are absolutely specific to the lost foam process, namely:
the formation of flaws due to gasified residues from the foam; and
the formation of carbon inclusions associated with oxides, as a result of contact between the liquid aluminum alloy and carbonaceous residues from the foam.
USSR Inventors' Certificate SU 1079353A discusses castings hardened in temporary sand-clay molds, and discloses that hardening the castings under increased pressure prevents porosity and results in high casting density. However, the increased pressure leads to mechanical burn-on due to the differential in pressure between the pressure acting on the surface of the melt and the pressure at the metal/mold interface, a differential which arises due to gas filtering through pores in the mold. In order to reduce burn-on of sand, SU 1079353 discloses that the pressure should be increased incrementally while the casting crystallizes, with the pressure being increased 0.1-0.2 MPa in each step at intervals of 0.2-0.4 seconds, with the pressure being held for a period of 1 to 5 seconds. The successive pressure increases are effected once the pressure in the system is equal to the pressure at the metal/mold interface and the pressure differential equals zero. The number of pressure increase steps is selected in such a way that the pressure differential at each step does not exceed a critical pressure and after increasing the pressure in each step, the pressure is held long enough to allow the pressure in the system to equalize with the pressure at the metal/mold interface.